A communication method of a terminal device moving within the Internet system is disclosed as Mobile IPv6 (Mobile Internet Protocol version 6) in a book titled “Detailed Illustration of IPv6 Expert Guide” written by Messrs. Hiroshi Ezaki, Yuji Sekiya, Hideaki Yoshifuji, and Tomohiro Ishihara, published from Shuwa System Co., Ltd. on May 25, 2002. The related article to the present invention appears in this book at page 96-99. This protocol allows a terminal device moving between dissimilar networks to communicate with others using the same address.
In Mobile IPv6 environment, when a terminal device leaves its home link to which this device primarily attaches, the device obtains prefix information transmitted from an access router of the present link, about the present foreign link, thereby generating a care of address to be temporarily used in the present foreign link from its primary home address and the prefix information.
Then the device transmits a binding update message to its home agent, thereby registering the care of address as a primary care of address with the home agent on the home link of the device. The home agent receives the binding update message from the terminal device, then generates or updates a binding cache which associates the home address with the care of address.
The home agent refers to the binding cache for receiving, as a proxy for the terminal device, a packet addressed to the home address on the home link of the terminal device, then encapsulates the packet for forwarding it to the care of address of the terminal device. The terminal device decapsulates the capsule forwarded by the home agent for receiving the packet originally addressed to its home address.
Mobile IPv6 has been designed with a mobile terminal device in mind; however, not with a mobile network, i.e. it is not assumed that a network as a whole moves among different networks. In such a case, a terminal device must carry out the process discussed above for itself.
Japanese patent application non-examined publication No. H09-172451 discloses the following communication method of a terminal device attached to a mobile network: A virtual IP (VIP) address, i.e. a home address that does not change while the terminal device moves among various networks, is given to the mobile router in addition to an address corresponding to a care of address, i.e. an Internet Protocol (IP) address that changes in response to the movement of the device. The home agent refers to an address management table (AMT) that associates the VIP address with the IP address, then forwards a packet to the IP address. This method is hereinafter referred to as a VIP method.
In the VIP method environment, a predetermined mobile router device to which terminal devices attach, and the home agent that manages the AMT forwards a packet addressed to the terminal device to the IP address of the predetermined mobile router, so that the communication is carried out. The terminal devices within the mobile network need not carry out the process involved by the movement, so that the movement involves a simpler process in the environment of the VIP method than in the environment of Mobile IPv6 method.
However, if the predetermined mobile router device, to which terminal devices within a mobile network attach, is switched over to another mobile router, the VIP method requires the respective terminal devices to carry out the process involved by the movement. To be more specific, plural mobile router devices are prepared on the mobile network as nodes between an access router on the Internet and the mobile network. If a defective is found in a present mobile router device or if a connection is lost between the Internet and the present mobile router device, the present mobile router device is switched to another mobile router device, so that the another one takes over the function of the present one. At this time, the predetermined mobile router loses the connection to the terminal devices, and the terminal devices move to this another mobile router device. Therefore, the respective terminal devices must carry out the process for associating their home addresses with care of addresses to their home agent. This process increases the load to the network, and it is impossible for all the terminal devices to register physical addresses, so that it takes a considerably long time for some terminal devices to register the physical address with the home agent.